1. Field of the Invention
The present invention relates to a process for improving the purity of a composition comprising a quaternary ammonium hydroxide.
2. Prior Art
Quaternary ammonium hydroxides such as tetramethylammonium hydroxide (TMAH) are used inter alia as a developer for photoresists in the manufacture of printed circuit boards and microelectronic chips and as a base in the production of 4-aminodiphenylamine (4-ADPA). Alkylated derivatives of 4-ADPA such as N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine (6PPD) are used as antidegradants in rubber compositions and rubber articles such as tires.
In said production of 4-ADPA, the basexe2x80x94which typically is in the form of an aqueous solutionxe2x80x94is recycled many times (hereinafter also referred to as recycle base). However, after a certain number of reaction cycles, the active content of the aqueous base solution has decreased to such an extent that it can no longer be used in the production process and either some of the recycle aqueous base solution is purged and replaced with fresh base solution or all of it is discarded as waste, which adds to the cost of the 4-ADPA and the 6PPD prepared therefrom. The present invention provides a solution to this waste problem. Also, with an increasing number of reaction cycles, the liquid-liquid separation of the aqueous base solution from the 4-ADPA-containing organic phase proceeds with greater difficulty.
When TMAH is used as the base, the purged/discarded aqueous recycle base solution contains inter alia various tetramethylammonium (TMA) salts, such as tetramethylammonium acetate, formate, chloride, carbonate, and oxalate as well as anilinexe2x80x94one of the starting materials for preparing 4-ADPA. It further contains small amounts of various other salts and other organic impurities.
Quaternary ammonium hydroxides are typically prepared by means of electrolysis. For example, TMAH may be prepared from tetramethylammonium chloride using a two-compartment electrolysis cell comprising an anolyte compartment containing an anode and a catholyte compartment containing a cathode, said compartments being separated by a cation selective membrane. Said membrane is also referred to in the art as a cation-exchange membrane. In this manufacturing process, the quaternary ammonium salt from which the quaternary ammonium hydroxide is prepared is charged to the anolyte compartment of the electrolysis cell.
It is also known in the art to improve the purity of mixtures comprising a quaternary ammonium hydroxide by electrolysis.
For example, U.S. Pat. No. 4,714,530 discloses a process for producing high-purity quaternary ammonium hydroxides by means of electrolysis using a two-compartment electrolysis cell equipped with a cation-exchange membrane in which an aqueous solution containing the quaternary ammonium hydroxide is charged to the anolyte compartment.
U.S. Pat. No. 5,389,211 discloses a process for improving the purity of organic or inorganic hydroxides such as quaternary ammonium hydroxides by means of electrolysis using an electrolysis cell comprising at least one intermediate compartment which is separated from the anolyte and catholyte compartments by at least two nonionic dividers and/or cation selective membranes. The mixture containing the hydroxide is charged to the anolyte compartment. It is mentioned that the catholyte and intermediate compartments may also contain organic or inorganic hydroxide prior to initiation of the electrolysis. It is mentioned that the purpose of charging purified hydroxide to the intermediate compartment is to avoid the build-up of impurities in that compartment (col. 12, II. 47-51).
The processes of U.S. Pat. Nos. 4,714,530 and 5,389,211 relate to improving the purity of in particular aqueous waste solutions of quaternary ammonium hydroxides which have been used as a developer for photoresists in printed circuit boards and microelectronic chips, which solutions typically contain significant amounts of halogen. The waste aqueous solutions comprising quaternary ammonium hydroxides which are obtained during the production of 4-ADPA, however, generally do not have a similarly high halogen content; they typically contain other anions as described above and organic impurities, in particular aniline.
In its primary embodiment, the present invention comprises a process for improving the purity of a composition comprising a quaternary ammonium hydroxide in comprising the steps of:
(a) providing an electrolysis cell which comprises an anolyte compartment containing an anode, a catholyte compartment containing a cathode, and at least one intermediate compartment, said at least one intermediate compartment being separated from the anolyte and catholyte compartments by cation selective membranes,
(b) charging water, optionally containing a supporting electrolyte, to the anolyte compartment, charging water, optionally containing a quaternary ammonium hydroxide, to the catholyte compartment, and charging the composition comprising the quaternary ammonium hydroxide to be purified to the intermediate compartment,
(c) passing a current through the electrolysis cell to produce a purified aqueous quaternary ammonium hydroxide solution in the catholyte compartment, and
(d) recovering the purified aqueous quaternary ammonium hydroxide solution from the catholyte compartment.
Other embodiments of the invention encompass various other streams charged to the electrolysis cell and their compositions, use of a three-component electrolysis cell, process conditions and details, types of cation selective membranes employed, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.